1. Field of the Invention
The present invention relates to an active spectrum sensing method of cognitive radio (CR) systems based on spectral correlation function (SCF). In this invention, the known pilots used for the primary users (PUs) are duplicated and reallocated in the subcarriers of CR signals. With this CR signal structure, the received signal of spectrum sensors will become correlated on the subcarriers when PU reoccupation occurs while the CR transmission is active, and thus PU activities can easily be detected by computing the spectral correlation function of the received signal.
2. Description of Related Art
In recent years, the wireless mobile communication technology provides users with a good spatial and temporal flexibility. However, the complexity of the wireless mobile channel characteristics, the limited radio spectrum resources, and emerging new businesses have been calling higher demands to wireless transmission technology for the flourishing development of wireless services.
With the rapid development of wireless communication technology, the spectrum scarcity problem has been gradually highlighted. The wireless spectrum resource has become one of the very precious resources. One survey of the U.S. Federal Communications Commission (FCC) shows that utilization of licensed spectrum in time and space is very low. For example, certain frequency bands have been allocated to a certain industry, but that industry does not use the entire bands. This example means that a considerable part of licensed bands is not utilized efficiently. The current utilization of the majority of the licensed spectrums is about 15% to 85%. Therefore, the FCC believes that the existing major problem is the static spectrum allocation of spectrum resources has not been fully utilized, instead of no spectrum is available.
The concept of CR was first proposed by Mitola in 1999. By cognizing environments, the CR system has the ability to improve the spectrum utilization by allocating unauthorized users (i.e., CR users) to the spectrum while the spectrum is idle at a specific time and in a specific space. This dynamic spectrum sharing can greatly increase the efficiency of spectrum utilization.
Spectrum detection system is essential to the CR technologies. The users of CR systems must monitor the spectrum changes in real-time so as to avoid any collision with an authorized user. Spectrum detection accuracy and reliability determine whether the normal communication for the authorized user can be assured. Currently available spectrum detection methods include matched filter detection, cyclostationary feature detection, and energy detection, etc. The matched filter detection needs to know the priori information of authorized user signals, such as modulation types, pulse shapings, and packet formats. The cyclostationary feature detection uses the spectral characteristics of the signals for detection, which has better detection performance at a low signal to noise ratio but with larger computational complexity. The energy detection is a traditional detection method; the greatest advantage of the energy detection is that the detector just needs to know the energy level of a signal in the frequency band to be detected without other a priori information of that signal. Therefore, the energy detection can be applied to the cognitive radio for a pre-crude detection to improve detection efficiency. However, the threshold of the energy detection is hard to set so that when the noise is uncertain it greatly influences the detection performance. Furthermore, the traditional energy detection only relies on the amount of energy as the base of judgment, which cannot distinguish between useful signals and interferences and noises, not mention to distinguish between the types of the received signals and modulation. The above defects will undoubtedly limit the use of energy detection in some circumstances.
The spectrum sensing technology is one of the key technologies for the cognitive radio systems. Periodic spectrum sensing architecture can provide reliable sensing sensitivity. Through introducing a quiet period between sensing signal frames, the primary users (PUs) can be protected from interfering. However, too many intra-frame quiet periods for the cognitive radio signals might deteriorate the quality of service (QoS) for the cognitive radio network. To solve this problem, it has been suggested that the intra-frame quiet sensing periods can be replaced by performing active spectrum sensing and data transmission simultaneously. A cyclostationary feature detector which is robust to noise can be used for active sensing. Detecting the features of pilot signals in the primary user signals effectively improves the detection sensitivity in the active detection. Due to the interference from the transmission of the cognitive radio network, the active detection has shortcomings such as finite sensing performance. That would be unchanged for the fact that the same spectrum interference causes the decrease in detection performance, even used in combination with the cyclostationary feature detection. Therefore, there is still a need of a novel method which can meet the user's need in practical use.